In an exhaust gas channel of a vehicle equipped with a lean-combustion engine such as a diesel engine or any other engine, a particulate filter is provided to collect particulate matter (PM such as carbonaceous particulates and other particulates) in an exhaust gas. This filter may be clogged if a large amount of PM is accumulated thereon. Thus, the amount of PM accumulated is estimated based on a difference between the pressures sensed by pressure sensors arranged in front of, and behind, the filter. When the amount of PM accumulated reaches a predetermined value, the temperature of the exhaust gas that reaches the filter is raised by controlling fuel injection of the engine (e.g., increasing the fuel or performing after injection) so as to burn the PM off. In order to burn the PM off efficiently, a diesel oxidation catalyst (DOC) is provided in the exhaust gas channel upstream of the filter, and a catalyst is loaded on walls of exhaust gas passages in the filter.
Specifically, the DOC is provided mainly for the purpose of increasing the temperature of the exhaust gas that flows into the filter to such a degree that the PM catches fire. The DOC allows for oxidizing and burning unburned HC and CO contained in the exhaust gas when the after injection is performed, and the heat generated by this reaction increases the temperature of the exhaust gas flowing through the filter. On the other hand, the catalyst provided on the walls of the exhaust gas passages in the particulate filter allows the PM accumulated on pores in the cell walls of the filter to catch fire more easily (i.e., reduces the temperature at which the PM catches fire), or ensures continuous burning of the PM so as to reduce the amount of PM released in the air, and shorten the time for regenerating the filter. This thus allows for reducing the amount of fuel injected for the regeneration. That is, this contributes to reducing a deterioration in fuel efficiency.
Such a DOC is proposed by, for example, Patent Document 1. Patent Document 1 discloses a configuration in which a catalytic material, containing an inorganic oxide selected from the group consisting of alumina, silica, titania, zirconia, ceria, and zeolite and a catalytic metal loaded on the inorganic oxide and selected from the group consisting of Pt, Pd, Rh, Ag, Au and Ir, is provided on a substrate of a wall-flow filter having gas distribution cells.
On the other hand, Patent Document 2 proposes a catalyst-loaded particulate filter in which a catalyst is provided on the walls of the exhaust gas passages in the filter as described above. Patent Document 2 discloses a catalyst-loaded particulate filter in which a catalytic layer, containing a Ce-containing composite oxide, a Ce-free, Zr-containing composite oxide, an activated alumina, and a catalytic metal, is provided on the walls of the exhaust gas passages in a filter for collecting particulates in the exhaust gas. According to Patent Document 2, Pt is loaded on the activated alumina.